Granular resins are supplied in the forms of emulsions, microgels, non-aqueous dispersion resins (NAD resins), powder resins and the like, and have been watched with keen interest in paint industries, especially in aqueous paints, high solid paints and powder paints, because of measuring up to the social requirements of economization of natural resources, energy saving and conservation of good surroundings.
However, such a resin is usually crosslinked so that the characteristics of the resin particles can be fully developed, and therefore when the granular resin is used alone, it is unable to yield a uniform or excellent film and the resulted film has a serious drawback of deficient film appearance.
Even when the granular resin is combined with a soluble type resin, there is a case that the viscosity of the mixture is unduly increased, as compared with that of said soluble type resin alone, due to the considerable interaction between the surfaces of said granules and the soluble type resin. Therefore, great care is often required in the actual use of such combination of resins.
Furthermore, since the characteristics of crosslinked resins are greatly influenced by the nature of surfactant used, crosslinking degree and combination of constituting monomers and the like, heretofore proposed crosslinked resin particles are hardly dispersible in such medium as aliphatic hydrocarbons, high boiling aromatic hydrocarbons, high polar solvents or the like,, and once they form agglomerates, hardly get loose back to the primary particles. Thus, considerable difficulties are always encountered in the actual application thereof.
It has also been well known to conduct the polymerization of acrylic monomers in multi-stages, thereby obtaining composite acrylic resin particles each having the so-called core-shell structure, the core being composed of crosslinked acrylic polymer and the shell being of crosslinked or non-crosslinked acrylic polymer. -;hen the shell portion is composed of non-crosslinked polymer, a comparatively good dispersion may be obtained with these particles in a soluble type resin or a solvent type coating composition. However, for a better ageing stability, the shell portion should preferably be chemically bonded to the crosslinked core resin, otherwise the non-crosslinked polymer in the shell portion will be gradually dissolved in said resin or organic solvent and the dispersion stability of the resin particles will be lost in time.
Under the circumstances, attempts have been made to effect graft polymerization in multi-stages, thereby chemically bonding the core and the shell layers, as for example, in Kamata et al. U.S. Pat. No. 4,362,845, Linder U.S. Pat. No. 4,393,172 and the like.
However, when the heretofore proposed composite resin particles were examined by dispersing them in butyl acetate, treating in a centrifugal machine to dissolve the non-crosslinked polymer into the solvent and measuring the remained particle weight, it was found that the grafting rate was generally of an extremely lower order. And, in fact, the dispersion stability of such resin particles in an organic solvent or resinous varnish was found to be rather poor.
It is, therefore, an object of the present invention to provide novel composite acrylic resin particles each comprising a particulate crosslinked acrylic polymer to which a number of substantially linear acrylic polymer chains are chemically bonded in a high grafting rate, which are free from the drawbacks possessed by the heretofore proposed composite resin particles.
An additional object of the invention is to provide novel composite acrylic resin particles which can be used either singularly or in combination form with other soluble resins customarily used in paint industries, to give excellent coating compositions with good aplication characteristics, and storage stability and capable of resulting in a uniform coating with excellent film appearance. Polymerization speed of an allyl compound, e.g. allyl acetate, is very low and its polymerization degree is likewise low. This is believed to be due to the occurrence of the so-called degradative chain transfer: EQU M.+CH.sub.2 =CH--CH.sub.2 OCOCH.sub.3 .fwdarw.MH+CH.sub.2 =CH--CH--OCOCH.sub.3
and the formation of resonance stabilized allyl radical of the formula: EQU CH.sub.2 CH CH--,
which is less reactive and hardly enters into a reaction with an acrylic monomer. See, R. C. Laible, Chem. Revs. 58 (1958) 807; Encyclopedia of Polymer Science and Engineering, vol 4, p. 779.
Therefore, in the preparation of the core portion of crosslinked acrylic polymer, when such polyfunctional monomer as conjugated diene is used as a crosslinking agent, it would be most probable that none of the double bonds which can be the initiation points of the subsequent graft polymerization can remain on the surface of the formed polymer particle, or even if they remain to some extent, they are stabilized by the aforesaid degradative chain transfer and cannot be used as active sites for the subsequent graft polymerization.
The inventors, after diligent studying on a way for chemically bonding linear polymer chains to the surface of particulate crosslinked acrylic polymer, have succeeded in attaining said objects by utilizing the selective addition of particular substituted ethylenic bonds and particular polymerizable monomers, and have arrived at the invention. Thus, according to the invention, the aforesaid objects of the invention can be attained with novel composite acrylic resin particles each comprising a particulate crosslinked acrylic polymer to which a number of substantially linear acrylic polymer chains are chemically bonded, prepared by a combination of steps of effecting an emulsion polymerization of a monomer mixture of
(A) at least one crosslinking monomer having in its molecule two or more radically polymerizable mono- or 1,1-di-substituted ethylenic unsaturation bonds, or a combination of at least two monomers each having a mutually reactive functional group and one or more radically polymerizable mono- or 1,1-di-substituted ethylenic unsaturation bonds, PA0 (B) at least one mono-functional polymerizable monomer other than an aromatic compound, and PA0 (C) at least one monomer having in its molecule one or more radically polymerizable mono- or 1,1-di-substituted ethylenic unsaturation bonds and one or more radically polymerizable 1,2-di-, 1,1,2-tri- or 1,1,2,2-tetra-substituted ethylenic unsaturation bonds, to obtain an emulsion of crosslinked polymer particles on which radically polymerizable 1,2-di-, 1,1,2-tri- or 1,1,2,2-tetra-substituted ethylenic unsaturation bonds still remain, and effecting a graft-polymerization of said polymer particles with a polymerizable aromatic compound together with other optional mono-functional polymerizable monomers. The linear polymer chain may be somewhat branched or crosslinked as desired. Therefore, in the specification and claims, the term "substantially linear" shall mean the polymer chains which are essentially of linear type polymer, admitting the presence of a degree of branching or crosslinking therein. PA0 (1) carboxyl group containing monomer as, for example, acrylic acid, methacrylic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid and the like, PA0 (2) hydroxyl group containing monomer as, for example, 2-hydroxyethyl acrylate, hydroxypropyl acrylate, 2-hydroxyethyl methacrylate, hydroxypropyl methacrylate, hydroxybutyl acrylate, hydroxybutyl methacrylate, allyl alcohol, methallyl alcohol and the like, PA0 (3) nitrogen containing alkyl acrylate or methacrylate as, for example, dimethyl aminoethyl acrylate, dimethyl aminoethyl methacrylate and the like, PA0 (4) polymerizable amide as, for example, acryl amide, methacryl amide and the like, PA0 (5) polymerizable nitrile as, for example, acrylonitrile, methacrylonitrile and the like, PA0 (6) alkyl acrylate or methacrylate as, for example, methyl acrylate, methyl methacrylate, ethyl acrylate, n-butyl acrylate, n-butyl methacrylate, 2-ethyl hexyl acrylate and the like, PA0 (7) polymerizable glycidyl compound as, for example, glycidyl acrylate, glycidyl methacrylate and the like, PA0 (8) .alpha.-olefin as, for example, ethylene, propylene and the like, PA0 (9) vinyl compound as, for example, vinyl acetate, vinyl propionate and the like, PA0 (10) reaction compounds of the abovesaid monomers as, for example, reaction compound of hydroxyl containing monomer (2) with isocyanate compound, reaction compound of carboxyl containing monomer (1) with glycidyl containing compound and the like.
The present composite acrylic resin particles are advantageously prepared by the following two steps.
1. Preparation of core portion of crosslinked acrylic polymer:
In this step, the following monomer mixture is polymerized in a conventional emulsion polymerization means i.e.,
(A) at least one crosslinking monomer having in its molecule two or more radically polymerizable mono- or 1,1-di-substituted ethylenic unsaturation bonds, or a combination of at least two monomers each having a mutually reactive functional group and one or more radically polymerizable mono- or 1,1-di-substituted ethylenic unsaturation bonds,
(B) at least one mono-functional polymerizable monomer other than an aromatic compound, and
(C) at least one monomer having in its molecule one or more radically polymerizable mono- or 1,1-di-substituted ethylenic unsaturation bonds and one or more radically polymerizable 1,2-di-, 1,1,2-tri- or 1,1,2,2-tetra-substituted ethylenic unsaturation bonds.
Examples of crosslinking monomer having in its molecule two or more radically polymerizable mono- or 1,1-di-substituted ethylenic unsaturation bonds are polymerizable unsaturated monocarboxylic acid esters of polyhydric alcohols, polymerizable unsaturated alcohol esters of polycarboxylic acids and aromatic compounds substituted with two or more vinyl groups.
More specifically, they are, for example, ethyleneglycol diacrylate, ethyleneglycol dimethacrylate, triethyleneglycol dimethacrylate, tetraethyleneglycol dimethacrylate, 1,3-butyleneglycol dimethacrylate, trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, 1,4-butanediol diacrylate, neopentylglycol diacrylate, 1,6-hexanediol diacrylate, pentaerythritol diacrylate, pentaerythritol triacrylate, pentaerythritol tetraacrylate, pentaerythritol dimethacrylate, pentaerythritol trimethacrylate, pentaerythritol tetramethacrylate, glycerol dimethacrylate, glycerol diacrylate, glycerol alloxy dimethacrylate, 1,1,1-trishydroxymethylethane diacrylate, 1,1,1-trihydroxymethylethane triacrylate, 1,1,1-trishydroxymethylethane dimethacrylate, 1,1,1-trishydroxymethylethane trimethacrylate, 1,1,1-trishydroxymethylpropane diacrylate, 1,1,1-trishydroxymethylpropane triacrylate, 1,1,1-trishydroxymethylpropane dimethacrylate, 1,1,1-trishydroxymethylpropane trimethacrylate, triallyl cyanurate, triallyl isocyanurate, triallyl trimellitate, diallyl terephthalate, diallyl phthalate and divinyl benzene.
Examples of the combination of monomers each having a mutually reactive functional group and one or more radically polymerizable mono- or 1,1-di-substituted ethylenic unsaturation bonds are epoxy containing ethylenically unsaturated monomer (e.g. glycidyl acrylate, glycidyl methacrylate and the like) and carboxyl containing ethylenically unsaturated monomer (e.g. acrylic acid, methacrylic acid, crotonic acid and the like). Various combinations of reactive groups are proposed as, for example, amine and carbonyl, epoxy and carboxylic anhydride, amine and acid chloride, alkyleneimine and carbonyl, organoalkoxysilane and carboxyl, hydroxyl and isocyanate and the like, and they are satisfactorily used in the present invention.
As the mono-functional polymerizable monomer other than an aromatic compound, the following may be used.
They may be used each singularly or in combination form. Examples of the monomer having in its molecule one or more radically polymerizable mono- or 1,1-di-substituted ethylenic unsaturation bonds and one or more radically polymerizable 1,2-di-, 1,1,2-tri- or 1,1,2,2-tetra-substituted ethylenic unsaturation bonds, are addition product of maleic acid and glycidyl acrylate, addition product of maleic acid and glycidyl methacrylate, addition product of fumaric acid and glycidyl acrylate, addition product of fumaric acid add glycidyl methacrylate, addition product of maleic acid monoester and glycidyl acrylate, maleic acid monoester and glycidyl methacrylate, addition product of fumaric acid monoester and glycidyl acrylate, addition product of fumaric acid monoester and glycidyl methacrylate, addition product of substituted maleic acid and glycidyl (meth) acrylate, addition product of substituted maleic acid monoester and glycidyl (meth) acrylate, addition product of substituted fumaric acid and glycidyl (meth) acrylate, and addition product of substituted fumaric acid monoester and glycidyl (meth) acrylate.
The emulsion polymerization may be carried out in a conventional way, using a polymerization initiator and an appropriate emulsifier. Particularly preferable emulsifiers are acrylic, polyester, alkyd or epoxy resin having in its molecule an amphoionic group of the formula: ##STR1## wherein R represents C.sub.1 to C.sub.6 alkylene or phenylene and Y.sup..crclbar. stands for --COO.sup..crclbar. or --SO.sub.3.sup..crclbar., as disclosed in Japanese Patent Application Kokai No. 129066/83.
In this first step of polymerization, only mono- or 1,1-di-substituted ethylenic bonds may participate in the reaction, giving crosslinked acrylic polymer particles still having unreacted 1,2-di-, 1,1,2-tri- or 1,1,2,2-tetra-substituted ethylenic unsaturation bonds on the surfaces thereof. 2. Preparation of the present composite acrylic resin particles each comprising a particulate crosslinked acrylic polymer to which a number of substantially linear acrylic polymer chains are chemically bonded: To thus obtained emulsion, a polymerizable aromatic compound is added and polymerization is continued to effect a graft polymerization between the remaining ethylenic unsaturation bonds and the polymerizable aromatic compound. Since 1,2-di-, 1,1,2-tri- and 1,1,2,2-tetra-substituted ethylenic bonds have a selective reactivity towards a polymerizable aromatic compound such as styrene, .alpha.-methyl styrene, vinyl toluene, t-butyl styrene and the like, a higher grafting rate can be attained with the aforesaid particulate crosslinked polymer coupled with the polymerizable aromatic compound. It is of course possible to use, besides the required polymerizable aromatic compound, other polymerizable monomers for the preparation of said linear polymer chains as desired. Any of the mono-functional polymerizable monomers hereinbefore stated under the column "preparation of core portion of crosslinked acrylic polymer" may satisfactorily be used. Furthermore, since a certain degree of branching or crosslinking is permissible according to circumstances, a limited amount of crosslinking monomer may be used together, as desired.
In any case, the shell portion of the present composite resin particles should be composed of substantially linear acrylic polymer and grafted to the crosslinked polymer core. Various desired properties may be given to the present composite resin particles by the selection of grafting monomers. For example, when the aforesaid monomers (1) or (3) are selected, the composite resin particles having carboxyl or amino containing polymer chains can be obtained, said particles having self-catalytic function in curing, high reactivity with an epoxy compound and being useful in an anionic or cationic electrodeposition use. When hydroxyl containing monomers are used, the resulted composite resin particles may be crosslinked with a melamine resin and/or isocyanate compound to give a tough coating. When the aforesaid monomers (4), addition products of hydroxyl containing monomers and monoisocyanate compounds or addition products of isocyanate containing monomers and monoamine compounds are used, it is possible to obtain the composite crosslinked resin particles with highly crystalline polymer chains which are useful in having structural viscosity and rheology control in a coating composition. It is also possible to carry on the linear polymer chains various functional groups and utilize the characteristic properties thereof. The present composite acrylic resin particles are excellent in dispersibilities in various solvents and resinous varnishes and possess self-film forming properties. Various functional polymers can be chemically bonded on the surface of the particulate crosslinked acrylic polymer. Therefore, the present composite acrylic resin particles are useful in various technical fields, including paint industries.